1. Field of the Invention
The present invention relates to an objective lens used when recording or reproducing information optically with respect to a disc-like optical information recording medium. The present invention also relates to an optical head including this objective lens. Further, the present invention relates to an optical information recording/reproducing apparatus that includes this optical head and records or reproduces information with respect to an optical information recording medium.
2. Description of Related Art
In recent years, to meet the demand for larger disk capacity, the technology for increasing the density of disk information has been developed actively. In connection with this, CD specifications, DVD specifications, BD (“Blu-ray Disc”, registered trademark) etc. have been suggested for a disk format.
In each of these specifications, an optical head is used for recording and reproducing information with respect to a disk. For achieving a higher density of the disk information, the wavelength of a semiconductor laser serving as a light source should be shortened and a numerical aperture (NA) of an objective lens should be raised in the optical head.
Moreover, in the optical head, compatibility for recording or reproducing has to be ensured among the disks of the above-noted specifications.
In response to the above, an optical head has been suggested in which the compatibility is ensured by one objective lens (in the following, the objective lens will be a single lens) (for example, see Journal of the Magnetics Society of Japan, 25, 449-450 (2001)). In the following, a conventional optical head achieving the compatibility will be described referring to the above-mentioned document.
FIG. 4 schematically shows how one common objective lens focuses light on information recording surfaces of two kinds of optical disks having different substrate thicknesses. In FIG. 4, numeral 101 denotes a Blu-ray Disc (hereinafter, referred to as “BD”), numeral 102 denotes a DVD, numeral 103 denotes an objective lens, and numeral 104 denotes a hologram element. Numeral 105 denotes a light beam having a wavelength of 405 nm, numeral 106 denotes a light beam having a wavelength of 650 nm, and numeral 107 denotes a center of the light beam. Further, WD11 indicates a working distance with respect to the BD 101, WD22 indicates a working distance with respect to the DVD 102, TH1 indicates a substrate thickness (0.1 mm) of the BD 101, and TH2 indicates a substrate thickness (0.6 mm) of the DVD 102. In FIG. 4, the left side of the center 107 of the light beam shows a cross-section illustrating how the light is focused on the BD 101, while the right side thereof shows a cross-section illustrating how the light is focused on the DVD 102.
The following is a description of how the light is focused on each disk. The objective lens 103 is designed based on optical specifications with respect to the BD 101, with a focal length of 2.5 mm, an optical numerical aperture (NA) of 0.75 and an outer diameter of 4.5 mm. The hologram element 104 is provided with a grid pattern so that zero-order diffraction light of the light beam 105 with a wavelength of 405 nm is focused on an information recording surface of the BD 101 via the objective lens 103 and +1-order diffraction light of the light beam 106 with a wavelength of 650 nm is focused on an information recording surface of the DVD 102 via the objective lens 103. Thus, the light beam 105 enters the objective lens 103 as a parallel light beam, travels across the working distance WD11 (=0.6 mm) and the substrate thickness TH1 and forms a desired light spot on the information recording surface of the BD 101. Also, the light beam 106 enters the objective lens 103 as a substantially divergent light beam, travels across the working distance WD22 and the substrate thickness TH2 and forms a desired light spot on the information recording surface of the DVD 102. In this case, the working distance WD22 is determined uniquely in terms of compatibility and, thus, WD22=0.5 mm.
From the optical standpoint, the conventional optical head has been described above. However, besides the bare disk, there also are disks contained in a cartridge (DVD-RAM specifications) as the DVD 102. Therefore, it also is necessary to ensure the compatibility with such disks.
The following description with reference to FIG. 5 is directed to a relative positional relationship between the DVD 102 contained in a cartridge and the conventional objective lens.
In FIG. 5, numeral 108 denotes a center axis (an axis of rotation) of the DVD 102, numeral 103a denotes an upper surface of an edge of the objective lens 103, numeral 121 denotes a cartridge for DVD-RAM, and numeral 121a denotes a bridge provided on an outer peripheral side of a shutter opening of the cartridge 121. R1 indicates an inner-bridge-surface radius (60.8 mm) from the disk center axis 108 to an inner peripheral surface of the bridge 121a, R2 indicates an outermost-track radius (59 mm) of the DVD 102, D1 indicates a bridge step (0.75 mm), which is a distance along the direction of the disk center axis 108 between a lower surface of the DVD 102 and a lower surface of the bridge 121a, and D2 indicates a height of a lens surface (0.15 mm), which is the distance along the direction of the disk center axis 108 between an uppermost point of the upper surface of the objective lens 103 and the upper edge surface 103a thereof.
However, when D3 is a dimension of a space along the direction of the disk center axis 108 between the lower surface of the bridge 121a and the upper edge surface 103a, the above-noted dimension condition yields
                    D3        =                              W            ⁢                                                  ⁢            D22                    +          D2          -          D1                                        =                              0.5            ⁢                                                  ⁢            mm                    +                      0.15            ⁢                                                  ⁢            mm                    -                      0.75            ⁢                                                  ⁢            mm                                                  =                              -            0.1                    ⁢                                          ⁢                      mm            .                              Thus, the space dimension D3 is negative, so that the bridge 121a and the objective lens 103 interfere with each other as shown in FIG. 5. In other words, the compatibility with the DVD 102 contained in the cartridge cannot be achieved.
In the above-described conventional optical head, the objective lens 103 designed on the basis of optical specifications with respect to the BD 101 and the wavelength dependent hologram element 104 realize the optical compatibility with the BD 101 and the DVD 102 that have different substrate thicknesses.
However, there is a problem that the compatibility with the DVD 102 contained in the cartridge according to the DVD specifications cannot be achieved because of the interference between the objective lens 103 and the bridge 121a of the cartridge 121.